Phenotypic divergence in widespread plants : genetic drift, selection and plasticity

This thesis presents studies that describe and explain phenotypic differentiation within several alpine plant species. The key elements that are addressed are threefold: (1) effects of neutral genetic drift, natural selection and phenotypic plasticity on phenotypic differentiation; (2) effects of glacial history, geography and climate on phenotypic differentiation and adaptation; (3) genetic structure and gene flow at small spatial scale. Combining all three elements, the aim of this thesis is to understand how a plant species' evolution towards its current state is affected at different spatial scales by neutral genetic drift and historical (i.e. glaciation-related) as well as more recent (i.e. postglacial) environmental influences.
To measure phenotypic differentiation in important plant traits, common garden experiments were performed with several alpine plant species (Campanula thyrsoides, C. barbata, Geum reptans) sampled from populations across the European Alps and Jura Mountains. Phenotypic differentiation was generally mirrored by molecular differentiation into distinct phylogeographic groups, which is explained by long-term survival in isolated glacial refugia. The results therefore suggest that glacial history affected not only the species' neutral genetic structure but also its phenotype. For some traits and in some regions, such differentiation could be explained as adaptation to the regional environment. For instance, the distinct phenology in Campanula thyrsoides, showing delayed flowering in the submediterranean southeastern Alps contrasting with early flowering at higher elevation in the other regions to the west, is clearly an adaptation to season length in the respective environments. Differentiation in various other traits could not be explained as adaptations and may therefore be due to drift alone.
Postglacial adaptation was detected when correlating trait values with altitude of origin. For instance, the negative correlation of altitude with plant height in Campanula thyrsoides, achieved without compromising flower production, is probably an adaptation to harsher conditions and to increased investment in roots.
Adaptation can also occur through phenotypic plasticity. In an experiment in which Campanula thyrsoides was grown in common gardens at three different altitudes, variability in the functional trait of specific leaf area could be dissected into a constitutive genetic part and a phenotypic plastic part.
At the local scale, populations of C. thyrsoides were considerably differentiated in neutral molecular markers, which could be due to founder effects in the recent past. Experimental studies showed that seed dispersal was also limited in the landscape. Within-population genetic diversity was found to be high and probably the result of strong self-incompatibility and outcrossing in this species. In line with this, a paternity analysis showed that pollen dispersal is well-mixed within the investigated population, but a substantial amount of pollen was derived from neighbouring populations in this specific landscape, indicating ongoing mixture.
To conclude, the studies described in this thesis showed that glacial history had strong effects on phenotypic differentiation, and that part of this differentiation is due to adaptation to past as well as current conditions, whether through constitutive genetic adaptation or phenotypic plasticity - though neutral genetic drift may also have a substantial contribution to differentiation. At the local scale, the heterogeneity of the European Alps and the particular autecology of alpine species may have contributed to local differentiation.